1096 GENERAL ANATOMY OR HISTOLOGY. 



THE BONE. 



Structure and Physical Properties of Bone. Bone is one of the hardest struct- 

 ures of the animal body ; it possesses also a certain degree of toughness and 

 elasticity. Its color, in a fresh state, is of a pinkish white externally, and a deep 

 red within. On examining a section of any bone, it is seen to be composed of two 

 kinds of tissue, one of which is dense in texture, like ivory, and is termed com- 

 pact tissue ; the other consists of slender fibres and lamellae, which join to form 

 a reticular structure ; this, from its resemblance to lattice-work, is called cancellous 

 tissue. The compact tissue is always placed on the exterior of the bone ; the can- 

 cellous is always internal. The relative quantity of these two kinds of tissue 

 varies in different bones, and in different parts of the same bone, as strength or 

 lightness is requisite. Close examination of the compact tissue shows it to be 

 extremely porous, so that the difference in structure between it and the cancellous 

 tissue depends merely upon the different amount of solid matter, and the size and 

 number of spaces in each ; the cavities being small in the compact tissue and the 

 solid matter between them abundant, while in the cancellous tissue the spaces are 

 large and the solid matter in smaller quantity. 



Bone during life is permeated by vessels and is enclosed, except where it is 

 coated with articular cartilage, in a fibrous membrane, the periosteum, by means 

 of which many of these vessels reach the hard tissue. If the periosteum is 

 stripped from the surface of the living bone, small bleeding points are seen, 

 which mark the entrance of the periosteal vessels ; and on section during life 

 every part of the bone will be seen to exude blood from the minute vessels which 

 ramify in it. The interior of the bones of the limbs presents a cylindrical cavity 

 filled with marrow and lined by a highly vascular areolar structure, called the 

 medullary membrane or internal periosteum, which, however, is rather the areolar 

 envelope of the cells of the marrow than a definite membrane. 



The periosteum adheres to the surface of the bones in nearly every part, 

 excepting at their cartilaginous extremities. When strong tendons or ligaments 

 are attached to the bone, the periosteum is incorporated with them. It con- 

 sists of two layers closely united, the outer one formed chiefly of connective 

 tissue, containing occasionally a few fat-cells ; the inner one, of elastic fibres of 

 the finer kind, forming dense membranous networks, which can again be separated 

 into several layers. In young bones the periosteum is thick and very vascular, 

 and is intimately connected at either end of the bone with the epiphyseal cartilage, 

 but less closely with the shaft, from which it is separated by a layer of soft 

 tissue, containing a number of granular corpuscles or " osteoblasts," in which 

 ossification proceeds on the exterior of the young bone. Later in life the peri- 

 osteum is thinner, less vascular, and the osteoblasts have become converted into 

 an epithelioid layer, which is separated from the rest of the periosteum in many 

 places by cleft-like spaces, which are supposed to serve for the transmission of 

 lymph. The periosteum serves as a nidus for the ramification of the vessels 

 previous to their distribution in the bone ; hence the liability of bone to exfolia- 

 tion or necrosis, when denuded of this membrane by injury or disease. Fine 

 nerves and lymphatics, which generally accompany the arteries, may also be 

 demonstrated in the periosteum. 



The marrow not only fills up the cylindrical cavity in the shafts of the long 

 bones, but also occupies the spaces of the cancellous tissue and extends into the 

 larger bony canals (Haversian canals) which contain the blood-vessels. It differs 

 in composition in different bones. In the shafts of adult long bones the marrow 

 is of a yellow color, and contains, in -100 parts, 96 of fat, 1 of areolar tissue and 

 vessels, and 3 of fluid, with extractive matter, and consists of a matrix of fibrous 

 tissue, supporting numerous blood-vessels and cells, most of which are fat-cells, but 

 some are "marrow-cells," such as occur in the red marrow, to be immediately 

 described. In the flat and short bones, in the articular ends of the long bones, 

 in the bodies of the vertebrae, in the cranial diploe, and in the sternum and ribs, 



